Modern communication systems often are assembled by interconnecting various arrays of electronic modules. Such electronic modules, commonly referred to as "cards", may be mounted in a housing, or they may be more or less open circuit boards. The modules commonly are loaded into racks, or "card cages", which facilitate the mounting and connection of the electronic modules.
Typically, card cages incorporate prefabricated panels which are assembled into a generally right-rectangular prism shaped frame having an open front. The back panel has electrical connectors, and the top and bottom panels have appropriately configured guides for receiving modules. In this manner, electronic modules can be slid into the cage and plugged into the back panel. The back panel may include a printed circuit board or various cable connections which facilitate interconnection of the electronic modules into a system.
The panels from which card cages are assembled are fabricated most commonly from sheet metal, and for certain applications, from aluminum, cold rolled steel, and stainless steel. The metal is punched and bent into the desired configuration. Thereafter, the panels are assembled and secured together by fasteners such as metal screws, nuts and bolts, rivets, and spot welding. Card cages which are assembled in this manner are sturdy, and the panels contact each other intimately so that the entire card cage may be grounded by a single connection anywhere on the cage.
Assembling such card cages, however, is relatively time consuming. A given fastening operation, for example, forming a rivet or weld, may not take very long when viewed in isolation; but there may be a great number of such connections which must be made. Especially if the modules emit high frequency electromagnetic radiation, a large number of connections are needed to block leakage of radiation through the seams of the card cage. For example, a thirty module card cage which is approximately 5".times.11".times.21" might require some thirty or more screws, rivets, spot welds or other fasteners to assemble the panels. Although there may be advantages in using one type of fastener as compared to another, the shear number of fasteners needed makes assembly of the card cage relatively slow whatever connector is used.
Sheet metal is usually the material of choice for card cages because of its cost advantage over other suitable metals, but sheet metal is relatively susceptible to corrosion. Corrosion is a particular concern when a card cage will be installed in environments which are not climate controlled. Moreover, corrosion can be exacerbated by galvanic reactions between the sheet metal of which the panels are composed and the fasteners which hold the panels together since the fasteners usually are made of dissimilar metals. Coatings typically are applied to protect the sheet metal from corrosion, but the integrity of a protective coating, if applied to the panels prior to assembly, may be damaged as fasteners are installed or formed. The fastener itself also may be relatively susceptible to corrosion. On the other hand, if a protective coating is applied after the panels are assembled, it may be difficult to coat all surfaces of the panels.
It also will be appreciated that shipping of card cages is made more costly because of their relatively large bulk in comparison to their weight. Thus, it may be desirable for some applications to ship panels in an unassembled state for assembly at a local distribution center or at the site where the card cage will be installed and used. Such "off-site" assembly of conventional card cages, however, is made more difficult or impractical because of the need for specialized equipment, such as riveters or welders, or simply because of the difficulty of forming in an relatively unsystemized setting the large number of fasteners needed in conventional card cages.
An object of the subject invention, therefore, is to provide a card cage which is sturdy, which can be easily grounded, and which otherwise meets the functional requirements for card cages, but which also is more easily and efficiently assembled. A related and more specific object is to provide such a card cage which is assembled with a reduced number of fasteners.
Another object of the subject invention is to provide such card cages which may be assembled in a manner which minimizes the likelihood of corrosion, and, in particular, is more compatible with the use of protective coatings for sheet metal.
Yet another object is to provide a card cage which may be shipped unassembled and is more susceptible to off-site assembly.
A further object of the subject invention is to provide such card cages wherein all of the above advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.